Is your question about Accounting Documents or Sales Order Documents?

Edits to Sales Orders never affect Accounting, it is just the corresponding Invoice that does.

Sales Order Documents:

LOCK the Sales Order and create (via attrs) an invisible condition on the UNLOCK button? 

See https://www.odoo.com/documentation/user/13.0/purchase/purchases/rfq/lock_orders.html for details (even though this link is for Purchase Orders, the same feature is available for Sales Orders)

You can see that the domain for "Today's Activities" we use context_today() to get the current date.

[('activity_ids.date_deadline', '=', context_today().strftime('%Y-%m-%d'))]

Accounting Documents:

This is the function of Lock Dates - see https://www.odoo.com/documentation/user/13.0/accounting/fiscality/fiscal_year/close_fiscal_year.html for a discussion